The vertebral column provides a strong, yet mobile central axis for the skeleton. The vertebral column is composed of a series of 31 separate bones known as vertebrae: seven cervical or neck vertebrae, 12 thoracic vertebrae, five lumbar vertebrae, five fused vertebrae that make up the sacrum, and two coccygeal vertebrae. Each vertebra is composed of a body anteriorly and a neural arch posteriorly. The arch encloses an opening, the vertebral foramen, which helps to form a canal in which the spinal cord is housed. Protruding from the posterior extreme of each neural arch is a spinous process and extending from the lateral edges of each arch are transverse processes. These bony elements serve as important sites of attachment of deep back muscles. The parts of the neural arch between the spinous and transverse processes are known as the laminae, and the parts of the arch between the transverse processes and the body are the pedicles. At the point where the laminae and pedicles meet, each vertebra contains two superior articular facets and two inferior articular facets. The superior articular facets form articulations, which are synovial joints, with the two inferior articular facets of the vertebra immediately above (or the skull, in the case of the first cervical vertebra). The pedicle of each vertebra is notched at its superior and inferior edges. Together the notches from two contiguous vertebra form an opening, the intervertebral foramen, through which spinal nerves pass. Differences in vertebral structure exist between the different spinal regions. For example, massive bodies and robust spinous and transverse processes characterize lumbar vertebrae. Lumbar vertebrae also contain small mammillary and accessory processes on their bodies. These bony protuberances are sites of attachment of deep back muscles.
In addition to various types of ligaments, such as supraspinous ligaments, interspinous ligaments, intertransverse ligaments, and ligamentum flavum, the bodies of adjacent vertebra are connected by cartilaginous joints known as intervertebral discs. Intervertebral discs are a fundamental component of the spinal motion segment, providing cushioning and flexibility. Each disc is composed of a central core of gelatinous material, known as the nucleus pulposus, and a surrounding series of fibrous rings known as the annulus fibrosis. In some individuals, the nucleus pulposus is forced out of the disc, or is herniated, which then affects the spinal nerves. Two ligaments connect the vertebral bodies anteriorly and posteriorly: the anterior longitudinal ligament, which is strong and robust throughout, and the posterior longitudinal ligament, which becomes thin and narrow in the lumbar region. This change in structure of the posterior longitudinal ligament is part of the reason that a majority of disc hemiations occur posteriorly in the lumbar region.
The spinal cord begins tat the level of the foramen magnum of the skull and ends at the level of the L1-L2 intervertebral joint. There it tapers to a cone-shaped ending known as the conus medullaris. All of the roots of the spinal nerves from L2 to the lowest coccygeal nerve pass caudal to the conus medullaris to exit at their respective intervertebral foramina. This mass of spinal roots within the spinal canal (in the subarachnoid space) is known as the cauda equina.
Degenerative disc disease, spinal stenosis, and posterior protrusion of a herniated lower lumbar intervertebral disc are diseases or conditions that can compress lumbar nerve roots, causing severe pain and/or physical dysfunction.
For conditions such as degenerative disc disease, spinal fusion is a surgical procedure designed to immobilize motion of the painful vertebral segments with the expectation of alleviation of pain associated with the degenerative disc disease. Spinal fusion may involve the removal of the intervertebral disc from the space between two vertebrae and subsequent incorporation of a cage (e.g., a rigid cage) into that space. In some cases, the rigid cage is hollow or a bone graft (e.g., autologous bone, synthetic bone) and/or other components are incorporated into the cage. The bone graft allows for bone growth between the two vertebrae (and through the cage), creating a bone fusion that inhibits motion. These rigid cages require support; hence spinal fusions also typically involve the insertion of pedicle screws and a metal rod or plate. For spinal stenosis, decompressive laminectomy, which features the removal of bone or tissue that causes narrowing of the spinal canal or squeezing of the spinal nerve roots, may be used to alleviate pain. Spinal fusion may be used as a stabilizing procedure following decompressive laminectomies.
The most commonly employed spinal fusion surgical techniques include: (i) posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), extreme lateral interbody fusion (XLIF), and anterior lumbar interbody fusion (ALIF). PLFI is a procedure done from the back and includes removing the disc between two vertebrae and inserting bone into the space created between the two vertebral bodies. Currently as part of the fusion procedure and in most surgical cases, bone is packed into small metal or carbon fiber containers called cages that act as additional support for the graft or the verterbral bodies. TLIF is similar to the PLIF as this procedure is also done from the back of the spine. However, the surgeon will make incisions that line up with the foramen vs. the middle of the back (as is done in a PLIF procedure). XLIF is an interbody fusion in which the approach is from the side. ALIF is done from the front and includes removing the disc between two vertebrae and inserting bone into the space created between the two vertebral bodies.
While fusion is the most common type of procedure performed on the lumbar region, there are risks associated with this approach: unnatural changes of motion pattern by immobilization; increased frequency of degeneration of adjacent segments; compensatory hypermobility of adjacent segments; and increased rate of spondylolysis (stress fracture) and spondylolisthesis (instability) of adjacent segments.
The only currently approved motion preservation solution for the lumbar region of the spine is disc arthroplasty (disc replacement). The challenge with disc arthroplasty is that the procedure must be done with an anterior approach, which is risky and complex. In addition, disc arthroplasty addresses the source of pain for a very small percentage of patients that present with lumbar spine pain. Prior to undertaking surgery for disc arthroplasty, a physician typically will utilize various tools such as X-ray, MRI and discograms to attempt to confirm that the source of the pain is actually attributable to the disc and not due to some other element of the spine. Confirming that the source of pain is the disc is a crucial indication since the anterior approach required for a disc arthroplasty involves additional risk to the patient due to the extensive nature of the surgery itself. Proving that a patient's spine pain is in fact due to disc failure is challenging and controversial and as a result, surgeons typically opt for other less risky procedures to alleviate the patient's pain.
The present invention features a mobile cage system for restoring motion kinematics of the spine that can be implanted via a posterior or lateral approach. The mobile cage system of the present invention provides sufficient support so as to eliminate the need for incorporation of a metal plate or rod and pedicle screws for enhanced stability. The mobile cage system of the present invention also prevents vertical collapse of the vertebrae and complete bone fusion; thus, the mobile cage system helps preserve intervertebral motion of the spine. Without wishing to limit the present invention to any theory or mechanism, it is believed that preservation of intervertebral motion of the spine is beneficial because motion-limiting methods, such as fusions, often result in degenerated arthritic or osteoporotic bone in adjacent vertebral area (e.g., adjacent segment disease). The present invention also eliminates the need for bone graft harvesting. Surgical procedures using the mobile cage system of the present invention may be easier and faster than spinal fusion surgeries.